MaskAlignment: Mask Highly Variable Regions of An Alignment

Description Usage Arguments Details Value Author(s) References See Also Examples

View source: R/MaskAlignment.R

Description

Automatically masks poorly aligned regions of an alignment based on sequence conservation and gap frequency.

Usage

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MaskAlignment(myXStringSet,
              type = "sequences",
              windowSize = 5,
              threshold = 1,
              maxFractionGaps = 0.2,
              includeTerminalGaps = FALSE,
              correction = FALSE,
              showPlot = FALSE)

Arguments

myXStringSet

An AAStringSet, DNAStringSet, or RNAStringSet object of aligned sequences.

type

Character string indicating the type of result desired. This should be (an abbreviation of) one of "sequences", "ranges", or "values". (See value section below.)

windowSize

Integer value specifying the size of the region to the left and right of the center-point to use in calculating the moving average.

threshold

Numeric giving the average entropy in bits below which a region is masked.

maxFractionGaps

Numeric specifying the maximum faction of gaps in an alignment column to be masked.

includeTerminalGaps

Logical specifying whether or not to include terminal gaps ("." or "-" characters on each end of the sequences) into the calculation of gap fraction.

correction

Logical indicating whether to apply a small-sample size correction to columns with few letters (Yu et al., 2015).

showPlot

Logical specifying whether or not to show a plot of the positions that were kept or masked.

Details

Poorly aligned regions of a multiple sequence alignment may lead to incorrect results in downstream analyses. One method to mitigate their effects is to mask columns of the alignment that may be poorly aligned, such as highly-variable regions or regions with many insertions and deletions (gaps).

Highly variable regions are detected by their signature of having a low information content. Here, information content is defined by the relative entropy of a column in the alignment (Yu et al., 2015), which is higher for conserved columns. The relative entropy is based on the background distribution of letter-frequencies in the alignment.

A moving average of windowSize nucleotides to the left and right of the center-point is applied to smooth noise in the information content signal along the sequence. Regions dropping below threshold bits or more than maxFractionGaps are masked.

Value

If type is "sequences" then a MultipleAlignment object of the input type with masked columns where the input criteria are met. Otherwise, if type is "ranges" then an IRanges object giving the start and end positions of the masked columns. Else (type is "values") a data.frame containing one row per site in the alignment and three columns of information:

"entropy"

The entropy score of each column, in units of bits.

"gaps"

For each column, the fraction of gap characters ("-" or ".").

"mask"

A logical vector indicating whether or not the column met the criteria for masking.

Author(s)

Erik Wright [email protected]

References

Yu, Y.-K., et al. (2015). Log-odds sequence logos. Bioinformatics, 31(3), 324-331. http://doi.org/10.1093/bioinformatics/btu634

See Also

AlignSeqs, IdClusters

Examples

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fas <- system.file("extdata", "Streptomyces_ITS_aligned.fas", package="DECIPHER")
dna <- readDNAStringSet(fas)
masked_dna <- MaskAlignment(dna, showPlot=TRUE)

# display only unmasked nucleotides for use in downstream analyses
not_masked <- as(masked_dna, "DNAStringSet")
BrowseSeqs(not_masked)

# display only masked nucleotides that are covered by the mask
masked <- masked_dna
colmask(masked, append="replace", invert=TRUE) <- colmask(masked)
masked <- as(masked, "DNAStringSet")
BrowseSeqs(masked)

# display the complete DNA sequence set including the mask
masks <- lapply(width(colmask(masked_dna)), rep, x="+")
masks <- unlist(lapply(masks, paste, collapse=""))
masked_dna <- replaceAt(dna, at=IRanges(colmask(masked_dna)), value=masks)
BrowseSeqs(masked_dna)

# get the start and end ranges of masked columns
ranges <- MaskAlignment(dna, type="ranges")
ranges
replaceAt(dna, ranges) # remove the masked columns

# obtain the entropy scores of each column
values <- MaskAlignment(dna, type="values")
head(values)

DECIPHER documentation built on Nov. 17, 2017, 1:01 p.m.